1. Field of the Invention
This invention relates generally to transmitter-receivers and, more particularly, relates to a transmitter-receiver to be used in broadband wireless access communications systems.
2. Description of the Related Art
A transmitter-receiver generally has a transmitting circuit for effecting transmission processing of sound information, data, and the like through an antenna, and a receiving circuit for effecting reception processing of sound information, data, and the like through an antenna. Since the transmitting circuit and the receiving circuit are to process information independently from each other, it is desirable that they do not interfere with each other. However, if a transmitting circuit and a receiving circuit are located physically close to each other, a mutual interference may occur due to an intrusion of a transmitting wave into the receiving circuit or of a receiving wave into the transmitting circuit. Since, in the case of a phone used in satellite communication systems, there may be a difference of 150 dB or more between the transmitting power and the receiving power, a deterioration in the receiving wave input from the antenna occurs if a transmitting wave at a high level intrudes into the receiving circuit, though a similar problem is not likely to occur if a receiving wave at a relatively low level sneaks in the transmitting circuit. For this reason, in a conventional terminal for a satellite communication system or the like, the transmitting circuit and receiving circuit may be formed on different assemblies and are provided with a sufficient shield.
However, as it is desirable to reduce the size and weight of communication equipment generally, it is inevitable that the transmitting circuit and the receiving circuit are located close to each other, on the same assembly.
In addition, narrowing of bands for the radio channel spacing is necessary from the viewpoint of effective utilization of frequencies, and a development of a low price transmitter-receiver achieving these by a simple construction is in demand. But to achieve the narrowing of bands for the radio channel spacing, a filter having a steep damping characteristic for suppressing an interference between neighboring channels becomes necessary. Therefore, circuit construction of the transmitter-receiver becomes complicated, resulting in problems such as an increase in price of the transmitter-receiver.
One attempt to overcome these disadvantages of the prior art has been the development of a transmitter-receiver as described in U.S. Pat. No. 5,689,819. There is described a transmitter-receiver which is located on a single assembly, with transmit and receive circuits being closely located to each other. However, the design illustrated therein suffers from certain disadvantages in that three separate local oscillator frequencies are utilized. These frequencies are selected to provide the necessary isolation between transmitter and receiver, but require associated hardware for each frequency which increases complexity and consumes valuable power source capacity in the hand-held communications products described.
From the above, it can be appreciated that it would be desirable to provide a transmitter-receiver circuit for communications applications which allows for close spacing between the circuit components while lowering costs and weight and which minimizes the amount of hardware and power consumption.